Ecological dynamics
The concepts of the Northern Wet Loess Terrace were developed from flatwoods communities within the Loess Plains of MLRA 134. The distribution of this site occurs to the west of the EPA Level IV Ecoregion, Flatwoods / Black Land Prairie Margins (65b) and should not be construed as representative of that ecoregion. Although, there is at least one overlap of a soil series, Adaton that occurs in both the Loess Plains and the Flatwoods ecoregions.
This ecological site is distributed across broad, level to nearly level flats on old fluvial terraces. A key characteristic of this site is the propensity of these poorly drained soils to perch water during wetter times of the year. Poor surface runoff and slow permeability contributes to saturated conditions from winter through spring in most years. This extreme wet condition through the first part of the year is reversed by mid- to late summer when evapotranspiration essentially removes moisture above the soils’ perching layer, which leads to very droughty conditions during summer and fall. The alternating wet – dry pattern of this site characteristically occurs in other “flatwoods” communities in the eastern United States (see Taft et al., 1995; Fralish et al., 1999). This moisture regime is often referred to as “hydroxeric” (or xero-hydric, depending on preference). It’s important to note that this site is generally above the modern floodplains of larger streams and rivers and receives very little overland flow. Exceptions occur where this site borders small streams and/or rivers and backflow moves over land.
Interspersed across the broad flats of this site are surface irregularities that include occasional, subtle mounds or micro-highs and distinct, shallow depressions. These microreliefs contribute to species and site diversity, overall. Depressional areas are designated as a separate ecological site, the Northern Ponded Loess Terrace, and is closely associated with the overall complexity of these poorly drained, terrace systems.
Because of the moisture regime and surface complexities, plant communities of this site are highly variable with some areas consisting predominantly of willow oak and others post oak. Species characteristic of both uplands and lowlands often co-occur within a small area (e.g., post oak, an upland species beside overcup oak, typically a wetland species). Areas that remain wetter for longer periods, such as shallow depressions, may support a greater abundance of pin oak, willow oak, and overcup oak. Conversely, areas that tend to dry more quickly may support a preponderance of post oak, hickory, and other upland species.
Some ecologists have typed or classified flatwoods based on the dominant and co-dominant plant species observed. Systems supporting a greater number of species having wet affinities (e.g., willow oak) have been classified as “wet flatwoods”, and those dominated by post oak and other drought-tolerant species are typed as “dry flatwoods” (NatureServe, 2009; M. Hines, personal communication). This provisional site represents both wet and dry versions because it is not clear which soils contribute to the observed “wet – dry vegetation gradient.” Very little work has been conducted in these dynamic and odd systems. As new information becomes available, one or more soil series provisionally associated with this site may warrant removal or new ones added.
Historically, fire may have been an important influence on this site. Low-intensity fires would have occurred when adjacent systems burned. Additional disturbance factors include severe ice-storm damage and windthrow from tornados and strong, straight-line winds (LANDFIRE, 2009).
The predominant land use activity on this site today is agriculture production with principal crops being soybean, corn, and cotton (USDA-NRCS, 2016). Some areas are in pasture or hayland with principal forage of bermudagrass, bahiagrass, and tall fescue. An additional use of this site is forest production. Overall, all uses on this site likely experience some limitations due to seasonal wetness, however forest production may have additional limitations due to seasonal dryness, particularly with respect to planted seedlings.
Following this narrative, a “provisional” state and transition model is provided that includes the “perceived” reference state and several alternative (or altered) vegetation states that have been observed and/or projected for the Northern Wet Loess Terrace ecological site. This model is based on limited reconnaissance, literature, expert knowledge, and interpretations. Plant communities will differ across MLRA 134 due to natural variability in climate, soils, and physiography. Some of the presented alternate states may warrant removal as additional information is learned and/or becomes available. Depending on objectives, the reference plant community may not necessarily be the management goal.
The environmental and biological characteristics of this site are complex and dynamic. As such, the following diagram suggests pathways that the vegetation on this site might take, given that the modal concepts of climate and soils are met within an area of interest. Specific locations with unique soils and disturbance histories may have alternate pathways that are not represented in the model. This information is intended to show the possibilities within a given set of circumstances and represents the initial steps toward developing a defensible description and model. The model and associated information are subject to change as knowledge increases and new information is garnered. This is an iterative process. Most importantly, local and/or state professional guidance should always be sought before pursuing a treatment scenario.
State 1
Hydroxeric Flatwoods
Exemplary examples of the full range of plant communities and ecological processes that were once commonplace on this ecological site no longer exist. Vestiges of this once vast system are primarily relegated to abandoned woodlots that have undergone numerous impacts since settlement. Still, there appears to be a close correlation between the species reported in earlier state reports (e.g., Loughridge, 1888; Lowe, 1921) and more recent accounts of extant woodlots (Bryant, 1999). One major difference between the historic community and present stands is the absence of fire, which is thought to have been an important disturbance factor (NatureServe, 2009).
Classifying this system with a single plant association name is extremely difficult, if not impossible, due to the variability of species dominance from stand to stand. One characteristic that appears to be similar throughout is the structural profile of a relatively open to moderately open understory, although this may vary due to local gap-scale dynamics. Some areas in West Tennessee are dominated by willow oak (on Adaton soils) while others support a mixed association of species that include post oak, willow oak, white oak, black oak, and hickory (on Routon soils). Some sites in western Kentucky have been observed to support a characteristic post oak flatwood with adjacent stands consisting of a mixture of post oak, black oak, cherrybark oak, hickory, with interspersed stems of overcup oak. Lowe (1921) reported the entrance of loblolly and shortleaf pine within flatwoods systems to the south in Mississippi. Such odd associations and combinations may be due to different inclusions of soil, interspersed micro-highs and depressions, former land use impacts (Bryant, 1999), latitude, or a combination of any or all of these factors.
A single community phase is recognized for representing the range of species occurring on this site. As additional information becomes available, revisions to this site may warrant significant changes. At a minimum, a new site will be developed for areas to the south in Mississippi where loblolly and shortleaf pine are components.
Of concern, portions of this site have incurred tremendous alteration to drainage. Any attempt to reestablish perceived reference conditions of a stand or a local site must first restore the natural hydrology of that location, which may entail removing drainage structures. If not, management may improve stand structure and even composition to a degree, but the site, overall, will remain in an altered state relative to reference conditions. Retaining drainage structures will directly influence the types of vegetation that colonize the site, which may be entirely different than what would occur, naturally.
Community 1.1
Post Oak – Mixed Oak Flatwoods
This community phase represents the compositional and structural complexity of stands supporting perceived reference conditions. With no intact example of a pre-settlement community remaining, this phase is arbitrarily chosen to represent the range of conditions that exist.
Composition of this community is quite variable ranging from post oak dominated flatwoods to a near monoculture of willow oak to a complex mixture of lowland and upland species. Where vegetation differs the most are within shallow depressions that are scattered throughout this site. (Note that the depressions represent a separate ecological site, the Northern Ponded Loess Terrace.) Species considered typical of this site include post oak, black oak, cherrybark oak, willow oak, water oak, shagbark hickory, mockernut hickory, pignut hickory, winged elm, along with occasional occurrences of white oak, southern red oak, overcup oak, pin oak, black cherry, and persimmon. To the south, loblolly and shortleaf pine are components and Lowe (1921) also reported blackjack oak as a component. Again, the combinations of one or more of the preceding species varies greatly from site to site. In general, species more typical of wet environments (e.g., overcup oak and pin oak) may be more associated with the interspersed, shallow depressions.
The understory is quite open with components consisting of red maple, winged elm, farkleberry, possumhaw, green hawthorn, and a herbaceous cover of little bluestem, Indian grass, broomsedge, woodoats, poverty oatgrass, agave, Indian physic, Willdenow’s croton, narrowleaf mountain mint, and a number of sedges and rushes.
State 2
Agriculture Production
Agriculture production is the dominant land use activity on this site, today, although production is somewhat limited due to seasonal wetness. Many areas in production have some form of drainage structures established due to seasonal saturation of the soil surface.
Community 2.1
Cropland
Crops may include soybean, corn, milo, and cotton.
State 3
Pastureland / Hayland
This state is representative of sites that have been converted to and maintained in pasture and forage cropland, typically a grass – legume mixture. For pastureland, planning or prescribing the intensity, frequency, timing, and duration of grazing can help maintain desirable forage mixtures at sufficient density and vigor (USDA-NRCS, 2010; Green et al., 2006). Overgrazed pastures can lead to soil compaction and numerous bare spots, which may then become focal points of colonization by undesirable plants or weeds. Soils exhibiting the core concepts of this site have a pronounced period of wetness followed by droughty conditions. Limitations may pertain mainly to periods of extreme wetness.
Planning or prescribing the intensity, frequency, timing, and duration of grazing can help maintain desirable forage mixtures at sufficient density and vigor (USDA-NRCS, 2010; Green et al., 2006). Because of the limitations of this site, grazing may be limited to small intervals within the appropriate season(s). It is strongly advised that consultation with State Grazing Land Specialists and District Conservationists at local NRCS Service Centers be sought when assistance is needed in developing management recommendations or prescribed grazing practices on this site.
Community 3.1
Select Forage/Species Mixture
This community phase represents commonly planted forage species on pasturelands and haylands. The suite of plants established on any given site may vary considerably depending upon purpose, management goals, usage, and soils. The limitations of this site may preclude some of the commonly planted mixtures. However, there is some indication that tall fescue, common bermudagrass, bahiagrass, and white clover may be adapted to this site (USDA-NRCS, 2006b).
Should active management (and grazing) of the pastureland be halted, this phase will transition to “old field” conditions, which is the transitional period between a predominantly open, herbaceous field and the brushy stage of a newly initiated stand of trees.
State 4
Timber Management
This state represents a broad range of management objectives, options, and stand conditions including woodlots allowed to grow or revert naturally; repeated single-tree harvests (often high-graded); carefully prescribed treatments; and conversion to a monoculture or single-species stand.
Various management or silvicultural methods can lead to very different structural and compositional results. For prescribed management options, methods are diverse, which include even-aged (e.g., clearcut and shelterwood) and uneven-aged (single tree, diameter-limit, basal area, group selection, etc.) approaches. Included within these methods is an option to use disturbance mechanisms (e.g., fire, TSI, etc.) to reduce competition and achieve maximum growth potential of the desired species. Inherently, these various approaches result in different community or “management phases” and possibly alternate states. The decision to represent these varying methods and management strategies into a single state and phase at this time hinges on the need for additional information in order to formulate definitive pathways, management actions, and community responses. Forthcoming inventories of this site will provide more detail on this state and associated management phases.
Of warning, there are significant timber management limitations due to alternating seasonal wetness and dryness; the latter often rendering the soil very hard and compact. These collective impacts could lead to high seedling mortality and/or low productivity of individual trees. Implementing this state or management option on this site should be carefully reviewed and considered.
Community 4.1
Mixed Oak – Pine (Various)
Some of the most desirable timber on this site consists of a few species of oak. However, productivity may be low, and there are moderate to severe limitations. Depending on the desired end product, management actions may differ. Management for oak dominant stands may be achieved by shelterwood and/or seed tree approaches. Managing for other hardwoods, and pine to the south, may only require timber stand improvement methods or artificial regeneration where other hardwoods predominate. Pine management should be relegated to southern portions of this site. Of caution, properties of these soils may limit the types of hardwoods that will survive and/or produce returns on the time and effort invested.
Finding the appropriate approach for a given stand and environment necessitates close consultation with trained, experienced, and knowledgeable forestry professionals. It is strongly urged and advised that professional guidance be secured and a well-designed silvicultural plan developed in advance of any work conducted.
State 5
Conservation
This alternative state is included to represent the range or breadth of conservation actions that may be implemented and established should other land uses be discontinued within a given location. Several actions may be chosen including the standard of establishing: native warm season grasses; suitable forbs for pollinators; select native trees to manage for forest or woodland conditions. If at all possible, the herbaceous species established should be derived from the “wild types” (genetic stock) from the Loess Plains or from adjoining ecoregions. This action would help preserve the unique genetic material from the area and would help to reintroduce the native herbaceous taxa back into a portion of their former range.
Community 5.1
Native Herbaceous or Woodland
This community phase represents the establishment of select native plants to meet conservation objectives on this site. As alluded to above, the best case scenario is the establishment of native species selected from the genetic stock of the Loess Plains or neighboring ecoregions. Herbaceous species suitable for establishing on this site include Indian grass, little bluestem, gama-grass, threeawn, wild oat grasses, panic grass, blazing star, evening-primrose, asters, sunflowers, goldenrod, tickseed, coneflowers, rattlesnake master, mountain mints, agave, milkweeds, sedges, among many others. Additional study is needed on this site before a complete set of plants can be generated. Key to the perpetuation and maintenance of this system is frequent fire, generally on a 1 to 3 year return interval (judgement based on early accounts of frequent burning; e.g., Loughridge, 1888).
Transition T1A
State 1 to 2
Actions include mechanical removal of vegetation and stumps; preparation for and establishment of cultivation (State 2).
Transition T1B
State 1 to 3
This pathway represents an attempt to convert the woodland community to pasture or forage production. Actions include clearing, stump removal, seedbed preparation, and the establishment of desired plants (State 3).
Transition T1C
State 1 to 4
This pathway consists of prescribed silvicultural activities specifically designed to meet stand compositional and production objectives (State 4).
Restoration pathway R2A
State 2 to 1
This pathway represents natural succession back to perceived reference conditions. The period required for this transition to take place likely varies by location and is dependent upon local site conditions. In some cases, a return to the reference state may not be possible without considerable management effort. That effort may involve exotic species control, restoration of the natural hydrologic regime of a given locality, and the reestablishment of components considered characteristic of the reference state.
Transition T2A
State 2 to 3
Seedbed preparation and establishment of desired forage/grassland mixture.
Transition T2B
State 2 to 4
This pathway represents prescribed management strategies for transitioning former cropland and/or cutover woodland to one that meets timber stand composition and production objectives. However, there are significant timber management limitations due to seasonal wetness and dryness; the latter often rendering the soil very hard and compact. For enhanced oak production, actions may include artificial regeneration and reduction of oak competition. Depending on species, seedling mortality could be high on this site. The final option of this pathway is the establishment of a pine monoculture or plantation. Establishment of the latter should be limited to southern portions of this site.
Transition T2C
State 2 to 5
This pathway represents the decision to discontinue cultivation/production and establish native grasses/forbs or trees on this site. This action also includes management activities to “guide” natural succession and conservation maintenance. Actions may include prescribed fire for maintaining and enhancing herbaceous establishment and herbicide treatments for controlling exotic species invasions and to ensure select tree establishment.
Restoration pathway R3A
State 3 to 1
This pathway represents natural succession back to perceived reference conditions. The period required for this transition to take place likely varies by location and is dependent upon local site conditions. In some cases, a return to the reference state may not be possible without considerable management effort. That effort may involve exotic species control, restoration of the natural hydrologic regime of a given locality, and the reestablishment of components considered characteristic of the reference state.
Transition T3A
State 3 to 2
Actions include removal of vegetation; herbicide treatment of residual plants; and preparation for crop establishment.
Transition T3B
State 3 to 4
This pathway represents natural succession of former pasture to non-managed “woods” or implementing prescribed management strategies for meeting timber stand composition and production objectives. However, there are significant timber management limitations due to seasonal wetness and dryness; the latter often rendering the soil very hard and compact. For enhanced oak production, actions may include artificial regeneration and reduction of oak competition. Depending on species, seedling mortality could be high on this site. The final option of this pathway is the establishment of a pine monoculture or plantation. Establishment of the latter should be limited to southern portions of this site.
Transition T3C
State 3 to 5
This pathway represents the decision to discontinue grazing/non-native forage management and establish native grasses/forbs or trees on this site. This action also includes management activities to “guide” natural succession. Actions may include prescribed fire for maintaining and enhancing herbaceous establishment and herbicide treatments for controlling exotic species invasions and to ensure select tree establishment.
Restoration pathway R4A
State 4 to 1
Natural succession over a period of time may transition a former timber-managed stand to one supporting reference conditions. Based on observations of some reference stands, a period greater than 50 years may be required, unless it was a former pine monoculture. Some question remains whether a return to reference conditions will occur in every situation, especially since some components may have been selectively culled from the stand. Management activities to aide recovery may include exotic species control and silvicultural treatment (State 1).
Transition T4A
State 4 to 2
Actions include removal of vegetation; herbicide treatment of residual plants; and preparation for crop establishment (State 2).
Transition T4B
State 4 to 3
Seedbed preparation and establishment of desired forage/grassland mixture (State 3).
Transition T4C
State 4 to 5
This pathway represents the decision to discontinue timber management or forest cover and establish native grasses/forbs or woodland/savanna on this site. This decision also includes the implementation of management activities to “guide” natural succession and conservation end goals. Actions may include prescribed fire for maintaining and enhancing herbaceous establishment and herbicide treatments for controlling exotic species invasions (State 5).
Restoration pathway R5A
State 5 to 1
This pathway represents natural succession back to perceived reference conditions. The period required for this transition to take place likely varies by location and is dependent upon local site conditions. In some cases, a return to the reference state may not be possible without considerable management effort. That effort may involve exotic species control, restoration of the natural hydrologic regime of a given locality, and the reestablishment of components considered characteristic of the reference state.
Transition T5A
State 5 to 2
This pathway represents the discontinuation of conservation practices and a return to production.
Transition T5B
State 5 to 3
This pathway represents the discontinuation of conservation practices and a return to pasture and/or hayland management entailing removal of vegetation, seedbed preparation, and establishment of desired forage/grassland mixture.
Transition T5C
State 5 to 4
This pathway represents natural succession of former pasture to non-managed “woods” or implementing prescribed management strategies for meeting timber stand composition and production objectives. However, there are significant timber management limitations due to seasonal wetness and dryness; the latter often rendering the soil very hard and compact. For enhanced oak production, actions may include artificial regeneration and reduction of oak competition. Depending on species, seedling mortality could be high on this site. The final option of this pathway is the establishment of a pine monoculture or plantation. Establishment of the latter should be limited to southern portions of this site.
